So now we know what John McCain really thinks of his running mate Sarah Palin – and that's not just because of the awkward body language between them during his concession speech in Phoenix, Arizona.

An exasperated McCain has been telling friends in recent weeks that Palin is even more trouble than a pitbull.

In one joke doing the rounds, the Republican presidential candidate has been asking friends: what is the difference between Sarah Palin and a pitbull? The friendly canine eventually lets go, is the McCain punchline.

McCain's joke is a skit on Palin's most famous line after she was picked as his surprise running mate. Palin delighted the Republican base when she said the only difference between a pitbull and a hockey mom was lipstick.

We owe the new glimpse into the tense McCain/Palin relationship to Sir Nigel Sheinwald, the British ambassador to Washington. Sheinwald recently wrote a lengthy assessment of McCain in a telegram that winged its way across the Atlantic to Whitehall.

The jaws of senior mandarins dropped when they read Sheinwald's account of McCain's thoughts on Palin which the ambassador reportedly picked up from a military friend of McCain's. The telegram was restricted to an even smaller group of people than usual for fear of another embarrassing leak. "We took one look at this and hid it away," one Whitehall source said.

Mandarins wanted to avoid a repeat of last month's embarrassing leak of Sheinwald's private thoughts on Barack Obama. My friend and colleague Ben Brogan reports that Sheinwald will be a nervous man today because those private thoughts got out.

In a memo to Gordon Brown, Sheinwald described Obama as a "decidedly liberal" man who "got diverted by his presidential ambitions".

Senior figures on both sides of the House of Commons are wondering how long Sheinwald can last in Washington because he is unlikely to be able to carry out the great Jonathan Powell instruction that a British ambassador should "get up the arse of the White House and stay there".

7.20pm update... The British Embassy said: "Nigel Sheinwald has not passed on any such anecdote about Senator McCain and Governor Palin. In fact he's had very warm relations with both parties throughout the election."